The sophistication and miniaturization of electrical apparatus have made possible features which have not heretofore been commercially achievable. For example, because low power and compact circuits are now available, cameras and other optical equipment can be completely electrically controlled. Moreover, with the integration of electrical and optical equipment, the convenience and ease of use is optimized.
In some types of optical equipment, it is most convenient, if not necessary, to optically provide the observer while using the equipment, indications of the conditions of various parameters associated with the equipment. For example, military personnel wearing night vision goggles would find it highly convenient to know if the batteries powering the electrical circuits of the goggle have become discharged beyond a predetermined voltage level. Moreover, the utmost advantage can be had with such equipment if a visual indication of the low voltage condition is presented to the goggle-wearer in the regular field of view, together with the object observed. In this manner, the goggle-wearer will automatically notice the visual indication without having to press a button or otherwise perform a deliberate act to check the voltage status of the battery.
Various approaches have been taken in the art for providing a user with visual indications of various parameters of the condition of electrical circuits of the equipment. For instance, in many photographic cameras it is common to mount a light source, such as a Light Emitting Diode (LED) internal to the camera, but out of the normal field of view. The person must then make a deliberate effort to look at a particular spot for the LED indication. While the size and space requirements of a camera are not particularly critical, this is not so, for example, in night vision equipment where the equipment must be lightweight and compact. Because of the compactness and lensing arrangement in night vision equipment, there is limited room internally for fixing light indicators indicating the status of the battery or of infrared lamps. Some attempts have been made to provide visual indicators within the goggle equipment, but again out of the field of view. This often required the user to rearrange the goggle or cant it so that the indicator could be seen.
There also exists image splitters with partially reflecting surfaces for integrating one ray bundle with another. U.S. Pat. Nos. 4,274,092 and 4,544,243 are exemplary of this approach. The problem attendant with the user of conventional image splitters is that the reflective surfaces thereof are only partially reflective so that light from one source can be transmitted through the splitter, while the light incident from the other source can be reflected therefrom. A significant amount of the light incident to the reflecting surface is transmitted therethrough, and thus not reflected. The intensity and quality of the image transferred is thereby reduced by the use of such image splitters.
It can be seen from the foregoing that a need exists for a method and apparatus for efficiently integrating a ray bundle comprising an object image, and a ray bundle comprising a visual indication of a desired electrical parameter.